Effects of limestone on pasture yields and the pH of two krasnozems in north-western Tasmania

Abstract

Ground limestone and molybdenum were applied to well fertilized, clover based, perennial pastures established on krasnozems with a pH of 5.5 at Elliott and Central Castra in north-western Tasmania. The experiments began in 1971 and continued until June 1976 at Central Castra and April 1979 at Elliott. Limestone was applied in increments of 2.5 t/ha over several years to achieve a maximum cumulative application of 12.5 t/ha at Central Castra and 15.0 t/ha at Elliott. These broadcast applications of limestone increased the pH of the surface 75 mm of the soils by 0.1 units for each tonne of limestone applied per hectare. Large increases in soil pH were recorded in the surface 150 mm of the soils when measured 18 months after the last limestone application but the increases were small (less than 0.2 pH units) below this depth. Limestone applications totalling 7.5-15.0 t/ha significantly increased pasture dry matter yields in more than half the harvests taken in summer, autumn and early spring but not in late spring. These increases, which ranged from 200-1200 kg/ha, are of potential agricultural importance since they occurred when feed supplies often restrict production from dairy herds grazing dryland pasture in northwestern Tasmania. The factor or factors limiting production on the unlimed plots have not been identified but, contrary to previous reports, the yield increases from the limestone applications were not due to an increase in the availability of molybdenum associated with the increase in soil pH.